1. Field of the Invention
The present invention relates to a method, circuit, and apparatus for detecting a phase difference between an optical signal and an electrical signal.
2. Related Background Art
A lot of PLL (Phase-Lock Loop) devices (phase comparator) and DBMs (Double-Balanced Mixers), both of which output a phase difference between two electrical signals, are commercially available as component devices. These component devices operate and output the phase differences between the electrical signals. For this reason, when a phase difference between a component of an intensity-modulated light and an electrical signal is measured, the optical signal must be converted into an electrical signal to measure the phase difference. Additionally, in these component devices, variations in intensity of an input electrical signal accordingly cause variations in phase difference output. Therefore, these component devices substantially need to be used at a constant input intensity.
As a phase difference detecting apparatus, a lock-in amplifier for outputting amplitude and phase difference information, a vector voltmeter mainly used in a high-frequency region, or a phase meter for outputting only a phase difference is known. These apparatuses are constituted to obtain an accurate phase difference even when the input intensity varies to some extent. In these apparatuses, however, when a phase difference between a light intensity-modulated component and an electrical signal is to be measured, the optical signal must be converted into an electrical signal (in some cases, the electrical signal is also amplified by an amplifier). Thereafter, the phase difference is measured. In this case, phase conditions must be sufficiently taken into consideration to constitute the apparatus.